In long distance optical communication systems it may be important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the optical transmission cable, faulty repeaters or amplifiers or other problems with the system.
Known monitoring techniques include use of a line monitoring system (LMS) including a network of loop back paths, e.g. high-loss loop back (HLLB) paths, within the system amplifiers/repeaters and/or other network equipment and line monitoring equipment (LME) that generates a test signal representing a pseudo random bit sequence. The LME may transmit the test signal with the information signals, e.g. in wavelength division multiplexed system. The test signal may be returned to the LME through the HLLB paths. The LME may include hardware and software components that separate the returned test signal from the data signals, and process the returned test signal to obtain data representing the HLLB loop gain imparted to the test signal in its propagation from the line monitoring equipment, through the HLLB and any intervening optical paths and amplifiers, and back to the LME. Significant deviations in HLLB loop gain may indicate a fault in the system.
A fundamental data set used by the LMS to determine if a system fault has occurred, e.g. in the optical fiber, repeaters, branching unit, etc., is the LMS baseline. In particular, an LMS baseline is a control measurement of gain data from each of the loop back paths made before the system goes into operation or before occurrence of faults or degradations in the system against which current measurements are compared to determine if new faults or degradations have occurred in the system.
Faults may be detected by the LMS using an Automatic Signature Analysis (ASA) algorithm. In general, ASA algorithms use gain data (e.g. differential gain data) of the LMS baseline versus gain data of a current measurement compared against a set of stored gain signatures to determine what fault may be present. One example of a system incorporating ASA is described in commonly owned U.S. Pat. No. 7,809,279, the teachings of which are hereby incorporated herein by reference.
Under certain conditions, such as after a system repair or if a system upgrade occurs that changes terminal line characteristics, the LMS baseline should be reset to a new LMS baseline measurement, i.e. the system should be “re-baselined.” Future measurements are compared to the new LMS baseline. Unfortunately, once a new LMS baseline is established any faults or degradations that may have been reported in comparison to the old LMS baseline will disappear as they will not be visible as compared to the new LMS baseline. Only new faults that occur after the new LMS baseline has been set will be detected by the ASA algorithms of the LMS.